PLC and/or DCS explained (more or less)

DCS: – “Distributed Control System”

Primary Controller

Secondary Controller

D C S

First, ‘Distributed’ implies that the functions are all distributed. These functions refer to the data collection, processing, controlling outputs, alarming, collecting historical data, etc. In a typical DCS system, you will find physical boxes that handle each of the above functions (well, almost). There is additionally an implication about geographical distribution, or that the functions can be distributed in different physical locations. This is a benefit in that prevents failure in one part of the system from affecting another part.

The word ‘Control’ implies the ability of the system to close the loop between an input measurement and a manipulated handle in the process. This means that the DCS shall have the capability for basic and advanced control algorithms.

The word ‘System’ implies that all of the above is connected as one contiguous system i.e. in simple words, one comprehensive system with systems and physical parts spread over potentially a wide area carrying out monitoring and control functions. A control system may have one or more PLCs (programmable Logic Controllers).

PLC: – “Programmable Logic Controller”

PLC Bank

PLCs

The PLC is a term applied in different ways. In large process facilities, there is a requirement that there be two systems–one for control while another for safeguarding. The plant-wide safeguarding portion is typically done by a safety-certified PLC. However, smaller PLCs could be used in smaller single loop control and safeguarding applications as well.

Historically a PLC forte was in discrete control of manufacturing processes. Most of the inputs and outputs for discrete control are binary, meaning they have only two states: on and off. Like a switch. Little processing power is needed for computing on/off signals so PLC tended to be very fast and are used in machine tool and other industries. The terminology “PLC” is interesting in itself. When they started becoming popular in the 1970s they were often called “relay replacers” since the logic for on/off type operations was done with relays arranged in a digital logic format. It was thought that calling them a computer would turn off many electricians and would scare people away from buying them. Manufacturers of the most popular PLCs include Allen-Bradley, Siemens, Modicon and a bunch of others.

Let me list down the advantages of DCS and PLC separately.

Advantages of DCS:

1. Excellent for regulatory control applications
2. A good fit for large/complex batch applications
3. Pre-packaged integrated system of hardware and software and hence low engineering time.
4. Robust multitasking system to handle large global common databases
5. Easy to interface with Enterprise applications.
6. Implementation of Dynamic simulation and Advanced Process application packages.
7. Future expansions i.e. scalability.
8. Migration for legacy systems.
9. DCS is application and solution driven plus industry aligned.
10. DCS systems are sold directly by the supplier and also Engineering of the DCS is done by the Supplier

Advantages of PLC:

1. Excellent for sequential control.
2. A good fit for small to medium batch applications.
3. Rugged, fast & low cost I/O.
4. Cost effective entry solution.
5. Ladder logic understood by many people.
6. PLC’s are sold by Distributors and engineered by System Integrators